<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    let arr1 = [1,2]
    let arr2 = [3,4]
    let arr3 = [[2,3,4],1]
    console.log(arr1.concat(arr3))
</script>
</body>
</html>